1. Field of the Invention
The present invention relates generally to a bar code reader and, more particularly, the present invention relates to a digitizer circuit for a bar code reader.
2. Discussion of the Related Art
Digitizers in bar code readers generally fall into one of two categories. One category, called diode thresholding digitizers, use diodes (usually in pairs) to set a threshold at which the digitizer will respond to modulations of the applied input signal. these digitizers will respond to a signal only when the change in magnitude of the input signal exceeds a forward voltage drop of a diode. Analog signals corresponding to the bar code symbol can be applied directly to the digitizer circuit, as in LS-7000 models from Symbol Technologies, Inc., the assignee of the present application, or a first derivative signal of the analog signal can be applied to the digitizer circuit.
In either case, diode thresholding allows a digitizer to digitize low levels of modulation of the analog signal. Hence, the first category of digitizers in bar code readers are useful for reading symbols with a large depth of focus for a given symbol density.
However, the features that allow these digitizers to provide a large depth of focus also make the digitizers sensitive to print defects on the symbol that cause relatively low levels of modulation of the analog signal. This is particularly a problem when a poor quality symbol is located near the waist of the laser beam (typically at about one to three inches from the nose of the scanner) where small defects are more easily resolved. As a result, hand-held laser scanners cannot read dot matrix symbols or other symbols with defects in this range using the first category of digitizers. However, these digitizers can read defective symbols by moving the scanner away from the symbol, causing the laser spot to grow larger. This renders the defects unresolvable.
The digitizer in the LS-2000 "Turbo" model of Symbol Technologies, Inc. is representative of the other category of digitizer, called a high-thresholding digitizer. High thresholding digitizers will not respond to modulations of the first derivative of the analog signal unless that modulation exceeds some pre-determined threshold. That threshold is a percentage of the height of modulation caused by the wide bars or spaces in the symbol being scanned.
High thresholding digitizers can have high enough thresholds to ignore printing defects that are small with respect to the laser spot. As a result, such digitizers can read poor quality symbols, even near the beam waist. Unfortunately, the ability to ignore printing defects makes this type of digitizer lack the large depth of focus of the diode thresholding digitizer.
Accordingly, conventional laser scanners that solve one problem create another. Digitizers sensitive enough to provide large depths of focus, give poor performance on defective symbols near the beam waist. Higher threshold digitizers work better on defective symbols near the beam waist, but have poor depth of focus.
There is a need for a laser scanner that can combine the strength of a sensitive diode thresholding digitizer and a high threshold digitizer. Such a scanner would be able to read a wider range of bar code symbols while maintaining high precision and accuracy.